1. Field of the Invention
The present invention relates generally to composite video systems and more particularly, to a method and apparatus for combining first and second video signals using an input lock map and one frame buffer to produce a composite video signal for display.
2. Description of Related Art
Chroma key is a mechanism long used by the studio television industry to isolate a desired portion of a live image. It is based on the notion of artificially constraining the colors in a scene so that a different set of colors occur inside a set of objects than in the background behind the set of objects. Typically, a saturated blue color is used as a backdrop in front of which objects are placed, since most natural objects do not contain saturated blue color.
A TV camera views the scene. The color components of the video signal from the TV camera are digitized and compared on a pixel by pixel basis to determine if the pixel's color falls within the region of the saturated blue color. For example, if the component colors are red, green and blue, a pixel that met the criteria would simultaneously have a small value for red, a small value for green, and a large value for blue. The result of this comparison generates a logical output which indicates whether the pixel belongs to the object or the background.
The intention is to replace those pixels belonging to the background with spatially corresponding pixels from another video source. Therefore, the logical output, referred to as a chroma key, can be used to select or key which of two live inputs is to be multilplexed on a pixel basis to the video output for display. What results is a composite image made up of a background scene from one video source with a live object from another video source being overlayed on top of it. For example, one pre-recorded video source might show the approaching ground filmed by a free fall parachutist. The other video source might be a TV camera pointed at a person sitting on a prop bomb in front of a blue backdrop. The composite chroma keyed output would show a person riding a bomb in free-fall towards the earth below.
This form of chroma key use for video overlaying video should not be confused with the more common video overlaying graphics form of chroma keying which only places a live video into a static window of a predefined shape.
If the video over video chroma key function is to be performed in front of a digital frame buffer, then one approach is to use two video digitizers and two digital frame buffers, with each of the digitizers consisting of a video decoder and a video rate analog to digital converter, to simultaneously process the two video sources so that they could be multiplexed into the digital frame buffers. However, there are many instances where video over video is not a sufficiently important feature to justify the expense of two video digitizers and two frame buffers. Examples of such instances include, personal computers and any video devices not involved in professional video productions. Accordingly, there is a desire to develop an overlaying two video signal system which does not require two costly video digitizers or two costly frame buffers.